The subject of the present invention is a process for the production of cellular porous monoliths of natural origin based on condensed tannins, the monoliths obtained by this process and applications thereof, as well as the emulsion and liquid foam enabling their manufacture.
“PolyHIPE” (Polymerized High Internal Phase Emulsion) materials, proposed for the first time in 1982 (EP 0060138), are obtained by polymerization of an emulsion termed HIPE (High Internal Phase Emulsion) composed on the one hand of an external or dispersant phase which is essentially constituted by polymerizable monomers and a surfactant agent in solution in a solvent, and on the other hand of an internal or dispersed phase which typically represents 74% or more of the total volume of the emulsion and which is essentially constituted by a solvent not miscible with the polymerizable monomers or with the solvent of the dispersant phase. After polymerization and removal of the solvent of the dispersed phase, open-cell materials are obtained the cells of which correspond to the imprint of the bubbles formed by that solvent in the course of the preparation of the emulsion and are interconnected by apertures of smaller size than themselves, commonly denoted by the term “pores”. On account of their properties, polyHIPE materials are the subject of growing interest, and their utilization has been proposed in numerous fields, among which there may be mentioned the manufacture of disposable absorbent articles, articles for thermal, acoustic, electrical or mechanical insulation, membranes, filters or even supports for inks, colorants and catalysts.
Foams are complex materials constituted by the dispersion of a gas in a condensed medium. Depending on the type of condensed medium, foams can be liquid or solid.
The formation of foams is a process which is regularly observed in nature when in particular gases are mechanically mixed into a liquid.
Liquid foams are materials which are in everyday use: surfactant foams, shaving foam, foamed milk, cappuccino or beer.
Solid foams are more difficult to find in nature. In general, they are produced from the liberation of a gas in a high viscosity liquid which hardens while the gas is escaping.
Solid foams are a class of materials generally characterized by their lightness and their cellular structure which ensure solutions that are advantageous from the point of view of their application. They can be classified according to the type of cells. Open-cell foams have very interconnected cells, and as a result their structure is very permeable and light. Closed cell foams have much greater strength than the former because the walls are not perforated and can therefore withstand greater compressive stresses.
Another classification of the solid foams is based on their physical properties:                Elastic foams have the property of being deformable while resuming their original shape when the stress which is applied to them disappears. The market for these foams is dominated by the polyurethane foams but latexes and EVA foams are also much used in specific sectors such as mattresses and sports accessories.        In contrast, rigid foams are materials which do not deform and the main applications of which are in the thermal and acoustic insulation of buildings. Rigid foams are sometimes utilized as shock-proofing in automobiles for their ability to absorb stresses and mechanical energy, and for their lightness. The most widely sold rigid foams are polyurethanes and phenolics.        
The vast majority of polyHIPE materials are obtained from polymers originating from petroleum resources which are not very environmentally friendly and the increasing scarcity of which entails a rise in production costs. Also, to decrease costs and for positioning as more environmentally friendly alternatives than these synthetic products derived from petrochemistry, and the production of which requires heavy energy expenditures, there has been a move towards “green” materials.
As regards the elastic foams, the commercial products mainly consist of polyurethanes, but some natural alternatives exist, in particular latexes, which are steadily increasing in the market. On the other hand, in the case of the rigid foams, practically no natural product is proposed as a replacement for the synthetic products.
“Green” materials, in particular those originating from biomass, should progressively replace their more expensive and less environmentally friendly synthetic equivalents. By biomass is meant all the lignocellulose products originating from agriculture and forestry (straw, fruit seeds and skins, wood and all other vegetable residues), but also derivatives thereof after separation by chemical, thermochemical or chemical-mechanical operations (lignin, cellulose, tannins, and all other furan and phenol compounds). The multifunctional nature of the materials originating from green chemistry allows an extraordinary variety of applications in the energy and environment fields.
Until now, few studies have been made and the only materials of the polyHIPE type manufactured from “green” material were produced from Kraft black liquor, a by-product of the paper-making industry (http://www.theses.fr/2011BOR14435). This liquor, heavily laden with various minerals, is difficult to utilize since the slightest variation in pH leads to the precipitation of the lignin. Moreover, the materials are extremely impure, which can be prejudicial for certain applications.
Therefore the inventors set themselves the problem of finding another material originating from biomass which would not have these disadvantages.
Well known for leather treatment, the tannins are polyphenolic components utilized by vegetables to defend themselves against insects and fungi. These substances are found in all vegetables in different percentages. The bark of trees in general contains the most significant quantity thereof, but tannin is present in the cytoplasm of all vegetable cells. The different woods store tannins in different areas of the vegetable: the pine (Pinus radiata), the oak (Quercus robur) and mimosa (Acacia mearnsii or mollissima) contain the majority of their tannins in the bark, and the gambier (Uncaria Gambir) in the leaves, whereas the chestnut (Castanea sativa) and the quebracho (Schinopsis balansae) store their tannins throughout their structure. The vegetable tannins can combine with proteins to give soluble or insoluble complexes. In spite of the differences in their compositions, they have a group of properties in common:                They precipitate proteins from their solution, in particular gelatin.        They give various colored lakes with the salts of heavy metals.        They precipitate with cationic colorants.        They are soluble in water to a greater or lesser extent and their solutions are always acidic.        They are amorphous and have no precise melting point.        
In terms of chemical composition, the distinction is made between two families of tannins: hydrolyzable tannins and condensed tannins or flavonoids.
The hydrolyzable tannins are constituted by simple phenolic substances: these are esters of gallic acid and of dimers thereof (digallic acid, ellagic acid) and monosaccharides (above all glucose). The hydrolyzable tannins are often divided into gallotannins, resulting in gallic acid after hydrolysis, or ellagitannins releasing ellagic acid after hydrolysis. They have already been used as partial substitutes for phenol in the manufacture of phenol-formaldehyde resins; nonetheless their utilization remains very limited in the adhesives field on account of their low reactivity with formaldehyde. On the other hand, the chestnut and tara tannins are very much utilized in the tanning industry.
In contrast to the hydrolyzable tannins, the condensed tannins are not decomposable by hydrolysis. On the contrary, when subjected to heating in an acidic medium they progressively polymerize and form amorphous anthocyanin pigments, of red color, or insoluble yellow-brown products, of high molecular mass, called pholbaphenes. On pyrolysis, the condensed tannins yield pyrocatechol.
The condensed tannins are constituted by flavonoid units classified into four entities (Porter, L. J.: The flavonoids. J. B. Harborne, Ed., Chapman and Hall, London, 1988)                tannins of the prodelphinidine type having a ring A of the phloroglucinol type and a ring B of the pyrogallol type (the base component is gallocatechin, Figure A),        
                tannins of the procyanidine type having a ring A of the phloroglucinol type and a ring B of the catechol type (the base component is catechin, Figure B)        
                tannins of the type prorobinetinidine type having a ring A of the resorcinol type and a ring B of the pyrogallol type (the base component is robinetinidol, Figure C), and        
                tannins of the profisetinidine type having a ring A of the resorcinol type and a ring B of the catechol type (the base component is fisetinidol, Figure D).        

The units of condensed tannins are generally linked by 4-6 and 4-8 bonds. The condensed tannins have a repetition of 2 to 8 flavonoid units.
When mixed with water, a hardening agent and a foaming agent, the tannins produce extremely light rigid foams. Their remarkable properties, similar and even superior to the commercial phenolic foams currently utilized in aerospace and marine applications, combine mechanical strength, thermal insulation, non-flammability and infusibility.
When subjected to other conditions, the tannins polymerize to give rigid gels, which are elastic to a greater or lesser extent. At a density equivalent to that of the rigid foams, materials the porosity of which is 1,000 to 10,000 times narrower are obtained. These are then no longer referred to as thermal insulators but as potentially thermal “super insulators”. The direct competitors of such ultra-light solids are the silica aerogels, very costly and originating from toxic chemistry. The aerogels from tannins are lighter, less expensive, non-irritant and opaque, which can make them still better in that they transmit very little or no infrared.
The pyrolysis of these two families of materials (foams and gels) leads to their equivalents in glassy carbon. The porous starting structure is retained, but the mechanical strength is improved with the heat treatment, at the same time as the resistance to thermal shock and chemical inertness.
Another valuable property became apparent: electrical conductivity. Not only are the uses of their organic precursors (sandwich composites, thermal and sound insulation, shock absorption, filtration of corrosive liquids or of molten metals) retained, but the carbon foams derived can also be utilized as porous electrodes, for electromagnetic shielding, heterogeneous catalysis, adsorption, etc. In addition, carbon aerosols from tannins have an excellent performance as supercapacitor electrodes. These devices, which serve for auxiliary electric power in trams, high-speed trains and other electric or hybrid vehicles, need further development and require properties of chemical inertness and porosity which the carbon gels are able to provide. About 5 times less expensive than their resorcinol-derived equivalents, the carbon aerogels from tannins are serious competitors for the storage of electrochemical energy.
The inventors have already described tannin-based foams prepared in a totally different manner, by physical and/or chemical foaming, that is to say the foam is formed by expansion and/or production of a gas in the formulation (Tondi G. et al. Carbon (2009), 47, No. 6, pages 1480-1492; Bioresource Technology (2009), 100, No. 21, pages 5162-5169; Zhao W. et al. Materials Chemistry and Physics (2010), 122, 175-182; Zhao W. et al. Materials Chemistry and Physics (2010), 123, 210-217; Basso M. C. et al. Advanced Materials Letters (2011), 2, 378-382; Ui X. et al. Maderas Ciencia y Tecnologia (2012), 14, 257-265; Li X. et al. Carbon (2012), 50, 2026-2036; Lacoste C. et al. Industrial Crops and Products (2013), 43, 245-250) and is not of the polyHIPE type.
It would therefore be desirable to have available a process for the preparation of materials which are either polyHIPEs, or foams, or hybrid products between polyHIPE and foams and which have high mechanical strength or, at the very least, sufficiently high for it to be possible genuinely to envisage their utilization in all the applications which have been proposed for this type of materials originating from petroleum resources and which makes use of the materials originating from biomass, which are inexpensive to produce.